CdSe Nanocrystals Research Articles

Effect of Gradient Alloying on Photoluminescence Blinking of Single CdSxSe1–x Nanocrystals

Alloyed CdSxSe1–x semiconductor nanocrystals (NCs) were obtained from a one-pot synthesis at reduced temperature with moderate quantum yield. Comprehensive structural characterizations of the CdSSe NCs reveal that the NCs have gradient alloyed structure, with Cd evenly distributed over the entire NC, Se rich in the center, and S rich in the outer region. This is due to the difference in the nucleation kinetics of S and Se precursors. Optical studies at the single NC level show that the NCs have reduced photoluminescence blinking, increased “on” time fraction, and good photostability, in comparison with CdSe NCs. The incorporation of sulfur composition in the alloy NCs improves surface passivation and in turn protects the NCs from (photo)oxidation. The gradual change in the NC composition from center to outer regions creates a smooth “interface”, compared to core/shell NCs. These factors lead to reduced nonradiative rates in the NCs, improving their emitting properties.

Role of Surface States in Photocatalysis: Study of Chlorine-Passivated CdSe Nanocrystals for Photocatalytic Hydrogen Generation

We examine the effects of chlorine-passivation of Cd surface atoms on photocatalytic H2O reduction by CdSe NCs. Transient absorption spectroscopy reveals that Cl passivation removes electron trap states in CdSe NCs, which is also reflected in an increase of photoluminescence quantum yield, e.g., from 9 to 22% after the Cl treatment. Size-tunable energy states in CdSe NCs enable the systematic investigation of surface defects and their effect on the photocatalytic hydrogen generation rate. It turns out that, depending on band-edge energy levels, the surface trap states may enhance or inhibit photocatalysis. Cl-treated CdSe NCs larger than 2.7 nm show a higher hydrogen evolution rate than untreated CdSe NCs of the same size as Cl treatment removes trap states with energy below the H2O reduction potential. In contrast, the same Cl treatment does not increase the photocatalytic rate of CdSe NCs smaller than 2.7 nm because both the conduction band edge and trap states are above the water reduction potential. T...

Raman analysis of chemical substitution of Cd atoms by Hg in CdSe quantum dots and rods

We investigate nanocrystals of ternary compounds CdXHg1−XSe with 0<X<1 using a Raman scattering technique at room temperature. We obtained the correlation between nanocrystal (NC) chemical composition and the frequencies of CdSe-like LO and the HgSe-like TO and LO-modes. It is shown that the crystalline structure of the original CdSe NCs used for Cd/Hg substitution, either zinc blende or wurtzite, strongly affects the structural properties of the resultant CdXHg1−XSe quantum dots and rods.

The Effect of Exciton-Delocalizing Thiols on Intrinsic Dual Emitting Semiconductor Nanocrystals.

The emissive properties of thiol-capped CdSe nanocrystals (NCs) with intrinsic dual emission are investigated through temperature-dependent photoluminescence (PL) measurements. We demonstrate the influence of thiols on the relative PL intensities of the core and surface emissive states, as well as on the observed Stokes shifts. A redshift of both the core and surface PL in comparison with phosphonate-capped NCs is consistent with recent work exploring the effect of thiols as excitonic hole-delocalizing ligands. This observation is consistent with prior reports suggesting that surface excitons originate from electrons bound to cadmium trap states.

Influence of nanocrystal concentration on the performance of hybrid P3HT:TBPO-capped CdSe nanocrystal solar cells

Hybrid solar cells based on the polymer poly(3-hexylthiophene) (P3HT) and colloidal TBPO-capped CdSe nanocrystals (NCs) have been studied using varying amounts of CdSe NCs. The power conversion efficiency of these devices increased monotonically from ηp = 0.05 % for 0 wt% NCs under white light illumination to 0.25 % for 40 wt% and then decreased to 0.07 % for 60 wt% NCs. The improvement in efficiency was mainly due to a remarkable increase in both the short-circuit current and open-circuit voltage, whereas the fill factor was less affected. In addition, the PL spectra of these devices showed a significant quenching suggesting rapid charge or energy transfer at the polymer–nanocrystal interface. The results indicate that capping with the smaller TBPO ligand did not considerably passivate the CdSe nanocrystal surface and the improvement in device performance could be mainly due to a pronounced PL quenching. The effect of using a relatively small TBPO compared to the commonly used TOPO ligand on the performance of these devices is discussed.

Intimate organic-inorganic nanocomposites via rationally designed conjugated polymer-grafted precursors.

Semiconducting organic-inorganic nanocomposites comprising the conjugated polymer poly(3-hexylthiophene) (P3HT) in intimate contact with CdSe nanocrystals were crafted by exploiting rationally designed P3HT-grafted cadmium precursors (i.e., Cd-P3HT complexes). The bifunctional ligand 4-bromobenzyl phosphonic acid (BPA-Br) that possesses two terminal functional groups at each end was employed, enabling the coordination of BPA-Br with Cd first to yield Cd-phosphonic acid complexes (Cd-BPA-Br) followed by the subsequent substitution of the bromide moiety into the azide (N3) group to form N3-functionalized Cd-phosphonic acid complexes (Cd-BPA-N3). Cd-P3HT complexes were then synthesized via a click reaction between Cd-BPA-N3 and ethynyl-terminated P3HT (P3HT-[triple bond, length as m-dash]). The success of the click reaction was confirmed by spectroscopic measurements. The morphology of CdSe nanocrystals (i.e., quantum dot and multi-branched) in P3HT-CdSe nanocrystal nanocomposites can be altered by tuning the concentration of Cd-P3HT complexes and the addition of excess Cd-BPA-Br (i.e., Cd-P3HT solely for the synthesis of CdSe quantum dots, and Cd-P3HT and Cd-BPA-Br at the ratio Cd-P3HT : Cd-BPA-Br = 1 : 1 for the synthesis of multi-branched CdSe nanocrystals). The photophysical properties of the resulting P3HT-CdSe nanocomposites were examined via absorption and photoluminescence studies. In comparison with P3HT-[triple bond, length as m-dash], the significant emission quenching of nanocomposites suggested the efficient charge transfer at the P3HT/CdSe interface. It is noteworthy that the implementation of judiciously synthesized Cd-P3HT complexes as precursors rendered the in situ synthesis of P3HT-CdSe nanocrystal nanocomposites, dispensing with the need for the use of insulating aliphatic ligands and tedious ligand exchange procedures for the preparation of functional polymer-tethered nanocrystals.

Direct synthesis of CdSe nanocrystals within a conducting metallopolymer: toward improving charge transfer in hybrid nanomaterials

CdSe nanocrystals with tunable sizes and distributions were synthesized directly from Cd(ii) ions coordinated to the conjugated backbone of a thiophene-based conducting metallopolymer. Using our seeded growth technique, the resulting nanocrystals are in direct electronic communication with the polymer, which leads to effective charge transfer between the donor and acceptor.

The effect of the neighboring impurity ion on the acceptor interlevel light absorbtion by CdSe nanocrystals

The effect of the neighboring impurity ion on the acceptor interlevel light absorbtion by CdSe nanocrystals

Temperature dependence of acoustic vibrations of CdSe and CdSe-CdS core-shell nanocrystals measured by low-frequency Raman spectroscopy.

We measure the temperature dependence of breathing-mode acoustic vibrations of semiconductor nanocrystals using low-frequency Raman spectroscopy. In CdSe core-only nanocrystals, the lowest-energy l = 0 mode red-shifts with increasing temperature by ∼5% between 77-300 K. Changes to the interatomic bond distances in the inorganic crystal lattice, with corresponding changes to the bulk modulus and density of the material, contribute to the observed energy shift but do not fully explain its magnitude across all nanocrystal sizes. Invariance of the Raman linewidth over the same temperature range suggests that the acoustic breathing mode is inhomogeneously broadened. The acoustic phonons of CdSe/CdS core-shell composite nanocrystals display similar qualitative behavior. However, for large core-shell nanocrystals, we observe a higher-order Raman peak at approximately twice the energy of the l = 0 mode, which we identify as a higher spherical harmonic-the n = 2, l = 0 eigenmode-rather than a two-phonon scattering event.

Surface modification of CdSeS nanocrystals for polymer hybrid solar cells

We report synthesis of Fluorene–Carbazole derivatives as caping agent for CdSeS nanocrystals to improve the performance of polymer nanocrystal hybrid solar cells.

Synergistic effect of carboxylic and amine ligands on the synthesis of CdSe nanocrystals

We demonstrate the drastic improvement in absorbance and photoluminescence (UV-PL) of CdSe nanocrystals (NCs) upon adding both a carboxylic acid and an amine ligand to the cadmium acetate green chemistry source.

Photoinduced transformations of optical properties of CdSe and Ag-In-S nanocrystals embedded in the films of polyvinyl alcohol

The results of investigation of photostability of the composites of CdSe and Ag-In-S nanocrystals (NCs) embedded in the films of polyvinyl alcohol (PVA) are presented. The films were studied by photoluminescence (PL), optical absorption, micro-Raman and X-ray diffraction methods. It is found that heating of the films to 100 °C promotes PVA crystallization and stimulates an increase of the PL intensity for the NCs of both types. The latter effect is ascribed mainly to the improvement of NC surface passivation by functional groups of PVA. The illumination with the 409-nm LED’s light enhances PL intensity for CdSe NCs and decreases it for Ag-In-S NCs as well as results in the darkening of the films. The color of the Ag-In-S-PVA film restores with time, while the change of the optical properties of the CdSe-PVA composite is irreversible. The possible mechanisms of the revealed effects, such as structural transformations at NC/PVA interface as well as the formation of new light-absorbing species are discussed.

Surface-enhanced Raman scattering by colloidal CdSe nanocrystal submonolayers fabricated by the Langmuir-Blodgett technique.

We present the results of an investigation of surface-enhanced Raman scattering (SERS) by optical phonons in colloidal CdSe nanocrystals (NCs) homogeneously deposited on both arrays of Au nanoclusters and Au dimers using the Langmuir–Blodgett technique. The coverage of the deposited NCs was less than one monolayer, as determined by transmission and scanning electron microscopy. SERS by optical phonons in CdSe nanocrystals showed a significant enhancement that depends resonantly on the Au nanocluster and dimer size, and thus on the localized surface plasmon resonance (LSPR) energy. The deposition of CdSe nanocrystals on the Au dimer nanocluster arrays enabled us to study the polarization dependence of SERS. The maximal SERS signal was observed for light polarization parallel to the dimer axis. The polarization ratio of the SERS signal parallel and perpendicular to the dimer axis was 20. The SERS signal intensity was also investigated as a function of the distance between nanoclusters in a dimer. Here the maximal SERS enhancement was observed for the minimal distance studied (about 10 nm), confirming the formation of SERS “hot spots”.

Tuning Equilibrium Compositions in Colloidal Cd1-xMnxSe Nanocrystals Using Diffusion Doping and Cation Exchange.

The physical properties of semiconductor nanocrystals can be tuned dramatically via composition control. Here, we report a detailed investigation of the synthesis of high-quality colloidal Cd1-xMnxSe nanocrystals by diffusion doping of preformed CdSe nanocrystals. Until recently, Cd1-xMnxSe nanocrystals proved elusive because of kinetic incompatibilities between Mn(2+) and Cd(2+) chemistries. Diffusion doping allows Cd1-xMnxSe nanocrystals to be prepared under thermodynamic rather than kinetic control, allowing access to broader composition ranges. We now investigate this chemistry as a model system for understanding the characteristics of nanocrystal diffusion doping more deeply. From the present work, a Se(2-)-limited reaction regime is identified, in which Mn(2+) diffusion into CdSe nanocrystals is gated by added Se(2-), and equilibrium compositions are proportional to the amount of added Se(2-). At large added Se(2-) concentrations, a solubility-limited regime is also identified, in which x = xmax = ∼0.31, independent of the amount of added Se(2-). We further demonstrate that Mn(2+) in-diffusion can be reversed by cation exchange with Cd(2+) under exactly the same reaction conditions, purifying Cd1-xMnxSe nanocrystals back to CdSe nanocrystals with fine tunability. These chemistries offer exceptional composition control in Cd1-xMnxSe NCs, providing opportunities for fundamental studies of impurity diffusion in nanocrystals and for development of compositionally tuned nanocrystals with diverse applications ranging from solar energy conversion to spin-based photonics.

Synthesis and Surface Chemistry of Cadmium Carboxylate Passivated CdTe Nanocrystals from Cadmium bis(Phenyltellurolate)

We report the synthesis of cadmium telluride nanocrystals from cadmium bis(phenyltellurolate) (Cd(TePh)2) and cadmium alkylcarboxylates (Cd(O2CR)2, O2CR = oleate, tetradecanoate). Cd(TePh)2 reacts quantitatively with Cd(O2CR)2 at 220 °C over 1 h with the concomitant elimination of diphenyl telluride (Ph2Te) and CdTe nanocrystals. The nanocrystal diameter approaches 3.0–3.2 nm at full conversion regardless of changes to the solvent, carboxylate chain length, heating conditions, and reaction concentration. Larger nanocrystals may be grown by slow addition of additional precursors to the crude product mixture. Isolated nanocrystals have carboxylate ligands (2.1–4.7 nm–2) that can be displaced along with excess cadmium ions using low concentrations of N,N,N′,N′-tetramethylethylenediamine (TMEDA) (1.5–280 mM). Using 1H NMR spectroscopy, we measure the extent of the displacement and show that the binding of Cd(O2CR)2 to CdTe is weaker than to CdSe nanocrystals of similar size. The weaker binding is proposed to ...

Evolution of the Single-Nanocrystal Photoluminescence Linewidth with Size and Shell: Implications for Exciton-Phonon Coupling and the Optimization of Spectral Linewidths.

The optimization of photoluminescence spectral linewidths in semiconductor nanocrystal preparations involves minimizing both the homogeneous and inhomogeneous contributions to the ensemble spectrum. Although the inhomogeneous contribution can be controlled by eliminating interparticle inhomogeneities, far less is known about how to synthetically control the homogeneous, or single-nanocrystal, spectral linewidth. Here, we use solution photon-correlation Fourier spectroscopy (S-PCFS) to measure how the sample-averaged single-nanocrystal emission linewidth of CdSe core and core/shell nanocrystals change with systematic changes in the size of the cores and the thickness and composition of the shells. We find that the single-nanocrystal linewidth at room temperature is heavily influenced by the nature of the CdSe surface and the epitaxial shell, which have a profound impact on the internal electric fields that affect exciton-phonon coupling. Our results explain the wide variations, both experimental and theoretical, in the magnitude and size dependence in previous reports on exciton-phonon coupling in CdSe nanocrystals. Moreover, our findings offer a general pathway for achieving the narrow spectral linewidths required for many applications of nanocrystals.

Сhiral recognition of cysteine molecules by chiral CdSe and CdS quantum dots

Here, we report the investigation of mechanism of chiral molecular recognition of cysteine biomolecules by chiral CdSe and CdS semiconductor nanocrystals. To observe chiral recognition process, we prepared enantioenriched ensembles of the nanocrystals capped with achiral ligand. The enantioenriched samples of intrinsically chiral CdSe quantum dots were prepared by separation of initial racemic mixture of the nanocrystals using chiral phase transfer from chloroform to water driven by L- and D-cysteine. Chiral molecules of cysteine and penicillamine were substituted for achiral molecules of dodecanethiol on the surfaces of CdSe and CdS samples, respectively, via reverse phase transfer from water to chloroform. We estimated an efficiency of the hetero- (d-L or l-D) and homocomplexes (l-L) formation by comparing the extents of corresponding complexing reactions. Using circular dichroism spectroscopy data we show an ability of nanocrystals enantiomers to discriminate between left-handed and right-handed enantiomers of biomolecules via preferential formation of heterocomplexes. Development of approaches for obtaining chiral nanocrystals via chiral phase transfer offers opportunities for investigation of molecular recognition at the nano/bio interfaces.

Generation of fluorescent CdSe nanocrystals by short-pulse laser fragmentation

A simple liquid-phase laser fragmentation approach, resulting in the rapid transformation of CdSe microcrystals into colloidal quantum dots (QDs), is presented. Laser fragmentation is achieved by irradiating a CdSe suspension in dimethylformamide with intense infrared, picosecond laser pulses followed by surface passivation with oleylamine or different types of phosphines. The generated QDs reveal perfect colloidal stability preventing agglomeration and precipitation, and show characteristic QD absorption and fluorescence characteristics, whereas their emission properties strongly depend on the surface states and applied capping ligands. These QDs show distinct photoemission under 405-nm single-photon and 800-nm multi-photon excitations in the 560- to 610-nm spectral region corresponding to the QDs size of about 1.5–2 nm in diameter which is confirmed by transmission electron microscopy.

Effect of Surface Stoichiometry on Blinking and Hole Trapping Dynamics in CdSe Nanocrystals

We measure the photoinduced carrier dynamics as the surface composition of CdSe nanocrystals is systematically varied from metal rich (∼80% surface Cd) to nearly stoichiometric (∼50% surface Cd). Using time-resolved optical spectroscopy, we determine that the luminescence lifetime is controlled by the rate of hole trapping at the newly exposed surface selenium atoms. However, the increased rate of the photoluminescence decay is not sufficient to explain the decreased photoluminescence quantum yield, and requires a growing proportion of nanocrystals in a dark or “OFF” state to explain the data. A global kinetic model is proposed that relates the fraction of selenium sites to the rate of hole trapping. A linear relationship between the rate of hole trapping and the fraction of exposed Se sites (xSe) is observed within the range of accessible stoichiometries (xSe = 0.5–0.2). Extrapolation to higher surface cadmium fractions suggests that not all Se sites serve as effective hole traps. These results explain t...

Fabrication of CdSe sensitized SnO2 nanofiber quantum dot solar cells

In the present study, we report a cost-effective quantum dot solar cells based on a combination of electrospinning and successive-ionic-layer-adsorption and reaction (SILAR) methods. CdSe nanocrystals are deposited on electrospun SnO2 nanofibers by SILAR method using CdCl2 as the cadmium source and Na2Se as selenium source. The as-prepared materials are characterized by spectroscopy and microscopy. CdSe deposited SnO2 electrodes are also characterized by spectroscopy and microscopy. Cells are fabricated with platinum (Pt)-sputtered FTO glasses used as the counter electrodes and polysulfide solution used as the electrolyte. The efficiency of the cells is studied for different number of SILAR cycles. Current density–voltage (J–V) measurements on a cell having CdSe deposition of 7 SILAR cycles and SnO2 coating area 0.25cm2 showed an overall power conversion efficiency of 0.29 % with a photocurrent density (JSC) of 5.32mA cm−2 and open circuit voltage (VOC) of 0.23V under standard 1Sun illumination of 100mWcm−2 (AM 1.5G conditions). This is improved by carefully coating SnO2 film without losing the structures. Also ZnS passivation layer is coated to obtain an improved efficiency of 0.48% with JSC of 4.68mAcm−2, and VOC of 0.43V.